Declare Phase 7c+7d page helpers in boundary.sx and register orchestration tests in app runner

Adds optimistic-demo-data, action:add-demo-item, offline-demo-data
to boundary spec. Adds orchestration test spec to in-app test runner
with mocked platform functions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/boundary.sx

@@ -79,3 +79,18 @@
   :params ()
   :returns "dict"
   :service "sx")
+
+(define-page-helper "optimistic-demo-data"
+  :params ()
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "action:add-demo-item"
+  :params (label)
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "offline-demo-data"
+  :params ()
+  :returns "dict"
+  :service "sx")